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Dealing with Uncertainties During Heat Exchanger Design

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Over the last thirty years much progress has been made in heat exchanger design methodology. Even so, the design engineer still has to deal with a great deal of uncertainty. Whilst the methods used to predict heat transfer coefficients are now quite sophisticated and take account of many physical factors, the results they yield are still inaccurate. Physical property information is required for the estimation of heat transfer coefficients. Available information is often of dubious accuracy. Even given accurate properties modern methods for the predictions of tube-side heat transfer coefficient can be expected to have an accuracy of only ± 10%. For the shell-side, higher errors (say, around ±15%) can be expected. Perhaps worst of all, comes the specification of fouling resistance (the allowance made for the thermal resistance presented by dirt layers deposited on the heat exchanger tubes). In most instances there is little science or understanding behind the specification of these resistances.
Traditionally there have been two approaches to dealing with these uncertainties: over-specification of fouling resistance; and, addition of 'design margin' (i.e. addition of extra surface area). There are cases in which both approaches are adopted. The engineer specifying the required duty provides a higher than necessary fouling resistance whilst the exchanger designer adds design margin! Both approaches result in 'over-design'.